Abstract
Microdinium Cookson & Eisenack, 1960 (type genus), and Eisenackia Deflandre & Cookson, 1955, have both been included in the cyst family Microdiniaceae. However, new information on the tabulation shows them to represent fundamentally different lineages. Microdinium displays a tabulation formula which was previously unknown among fossils and which resembles and suggests the affinities of Microdinium with representatives of the modern family Cladopyxidae (Kofoid 1907) Balech, 1964. Eisenackia displays a Gonyaulax-type tabulation and is transferred from the Microdiniaceae to the Gonyaulacaceae.